The Ewing's sarcoma family of tumors (ESFTs) contains a characteristic translocation that joins the EWS gene on chromosome 22 to an ets family gene, FLI1, located on chromosome 11, t(11;22). This novel transcript is translated to become the EWS/FLI1 fusion protein, which functions as an oncogenic transcription factor. Another key component of transformation in ESFT is the insulin-like growth factor (IGF) signaling system. The IGF type I receptor (IGF-IR) is required for EWS/FLI1 transformation of fibroblasts. Insulin-Receptor Substrate-1 (IRS-1), the key substrate of IGF-IR is regulated by tyrosine phosphorylation and dephosphorylation. EWS/FLI1 transformation is associated with a basal reduction in tyrosine phosphorylation of IRS-1; until now, the phosphatase responsible for this has eluded discovery. We have identified a protein tyrosine phosphatase (PTP) PTPL1 (aka, PTP-BAS, human PTP1E, PTPN13, and later FAP-1) that we believe could be responsible for the connection between EWS/FLI1 and IGF-IR signaling. We therefore hypothesize that the EWS/FLI1 fusion protein functions as an oncogene by inducing PTPL1 expression. We further hypothesize that PTPL1 tips the balance of signaling in ESFT to favor tumor cell survival and transformation. In addition to IGF-IR, PTPL1 has been reported to modulate other key ESFT signaling pathways including Fas and p75NTR. Our specific aims will (i) identify how PTPL1 contributes to the ESFT malignant phenotype by evaluating growth, survival and tumorigenesis in ESFT clones with reduced PTPL1 levels, (ii) determine relevant PTPL1 clinical pathways and if PTPL1 pathway signatures in ESFT patient tumors affects clinical response to therapy and overall survival (training set, n=100, validation set n>100), and (iii) establish the nature of PTPL1 regulation by EWS/FLI1 in ESFT cell models. We are enthusiastically pursuing how the phosphatase PTPL1 regulates ESFT biology as the logical extension of our current IGF-I studies. One of the more important findings we describe in our Preliminary Data is that reduction of PTPL1 severely reduces ESFT colony formation in soft-agar. PTPL1 is a novel regulator of IGF-IR and Fas pathways. Clearly, if PTPL1 is validated as a supportive oncoprotein and the pathways modulated by PTPL1 are identified, patients with many other tumors that rely on IGF-IR signaling including breast carcinoma could benefit from our findings.